Lac Operon
Process !!
In
E.coli,It’s
LAC OPERON
The first example of gene regulation system was the lac
operon, in which protein involved in lactose metabolism are
expressed by E.coli only in presence of lactose & absence of
glucose.
In 1961, Francois Jacob & Jacques Monod described the “operon
model” for the genetic control of lactose metabolism in E.coli
THE LAC OPERON PROCESS
1. In the lac operon, the lacZ, lac Z, lac A genes have a
common promoter lac P & transcribed together.
2. At the upstream of the promoter regulator gene lac I
present with its own promoter.
3. Lac I gene transcribed a short mRNA that translated into
repressor protein, which consist four identical
polypeptides and has two binding site, one binds to
allolactose & other binds to DNA.
4. Immediately upstream of the structural gene present lac
promoter lacP.
5. RNA polymerase binds to the promoter & moves down the
DNA molecule, transcribing the structural genes.
In the absence of lactose
1. The repressor binds to the lac operator site lac O.
2. Then the binding of RNA polymerase is blocked &
transcription is prevented.
In the presence of lactose
1. When lactose present, some of its converted into
allolactose.
2. Allolactose binds to the repressor & making the protein
inactive.
3. The repressor protein cannot bind to the operator, & the
binding of RNA polymerase is no longer blocked.
4. Transcription of lacZ, lacY, lacA takes place and lac
enzymes are produced.
CATABOLITE REPRESSION / GLUCOSE EFFECT
Lactose alone can activate the operon. Presence of glucose
prevents the induction of the lac operon.
E.coli cells metabolize glucose more easily than lactose.
Besides it E.coli cells keep the lac operon turned down as
long as glucose is present. This selection in favour of
glucose metabolism & and against use of other energy sources
has long attributed to the influence of some break down
product or catabolite of glucose, known as catabolite
repression / glucose effect.
POSITIVE CONTROL OF LAC OPERON
1. The positive controller of the operon is a complex
composed of two parts cAMP (cyclic AMP) & a binding
protein called CAP (catabolite gene activator protein)
for catabolite activator protein.
2. Lac promoter has two parts, the CAP binding site on the
left & RNA polymerase binding site on the right.
3. The protein binds cAMP & the complex binds to the lac
oparator region and helps RNA polymerase to bind there
4. When CAP/cAMP binds, it causes DNA to bend at 90° angle.
5. This bending helps RNA polymerase bind & making it
easier for the RNA polymerase to separate the two DNA
strands forming an open promoter complex.
6. cAMP/CAP binds to the upstream part of the promoter and
facilitates binding of RNA polymerase to the downstream
part. This enhances transcription of the operon.
When glucose is present in the medium, catabolite repression
occurs. In catabolite repression, the expression of the
lactose operon is turned off even if lactose is present in the
medium.
This occurs because the presence of glucose causes that amount
of cAMP in the cell to be greatly reduced.
So insufficient CAP-cAMP complex is available to facilitate
RNA polymerase binding to the lac operon promoter &
transcription is lower significantly, even though repressors
are removed from the operator by the presence of allolactose.
When glucose is present in high concentration, the cells c-AMP
concentration is low, when glucose concentration decreases,
the cellular concentration of c-AMP is necessary for
activation of the lac operon.
This is the general notes on Lac Operon.
Thank you for reading.